The evolutionary developmental psychology about the secular trend

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The material collected in February2011 consisted of a random sampling of45，830schoolgirls (9-15 years old). The mean age at menarche of Japanese schoolgirls was found to be 12.189 years (12 years and 2.3 months.) In comparison with the mean age of 2008, the mean age had not significantly changed. The mean menarcheal age in Japan was thought to be decreasing since the 1990s. The trend towards early maturation in Japan had obviously leveled off．The age of menarche serves, as well as an evolutionary index, as a reliable instrument with which the health practice and socio-economical difference of each region may be measured.